Showing popularity through time with stack

D3's official docs say:

"The stack layout takes a two-dimensional array of data and computes a baseline; the baseline is then propagated to the above layers, so as to produce a stacked graph."

Not clear at all, but I am hard pressed to come up with anything better. The stack layout calculates where one layer ends and another begins. An example should help.

We're going to make a layered timeline of donations over the 5-year sample, stretching as far back as 2010, with the width of each layer telling us how many donations went where at a certain time. This timeline is called a streamgraph.

To label layers, we're going to create a mouseover behavior that highlights a layer and shows a tooltip with the donation ...

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